1. Field of the Invention
The present invention relates to phase-locked loop oscillators and especially to those designed to generate the clock signals of an electronic circuit made on an integrated circuit chip.
The performance characteristics obtained today by integration techniques enable the integration, on an integrated circuit chip, of electronic circuits based on complex combinations of logic circuits performing sequences of operations at very high rates, known as clock rates. These clock rates can no longer be distributed synchronously on printed circuit boards. This is because the time taken by the clock signals to travel from a single location to different locations of the integrated circuit chips and the dispersion between these travelling times are no longer negligible as compared with the period of these signals. This impossibility of achieving a synchronous distribution, on printed circuit boards, of the very high clock rates required by the electronic circuits integrated chips raises a problem when it is sought to synchronize various electronic circuits integrated into various chips distributed on one or more printed circuit boards.
2. Description of the Prior Art
To resolve this problem, there are known ways for the distribution, on the printed circuit board or boards, of a synchronization signal that is a sub-multiple of the clock signals of the electronic circuits integrated into the chips and for the integration, at the periphery of each chip, of a phase-locked loop oscillator generating the clock signal proper of the electronic circuit integrated into the chip considered. This clock signal is phase-locked with the synchronization signal and has a rate that is a multiple of this signal and is adapted to the performance characteristics of the electronic circuit integrated into the chip.
The technique of the integration of electronic circuits based on logic circuits is evolving very rapidly. Various integration technologies, which are often mutually incompatible, are sharing the market at any given time. Some are becoming obsolete while others are emerging in the course of time so much so that it is not rare, in order to ensure the permanence of an electronic circuit based on logic circuits integrated into a chip, to have to change the integration technology.
This changing of the integration technology does not raise any particular problems for an electronic circuit based on logic circuits inasmuch as, for each integration technology, it is the common practice to develop libraries of logic circuits which only need to be drawn upon in order to obtain integration in an efficient way. Furthermore, the connection pads that are dedicated to them on the periphery of a chip are not specialized. This means that it is always possible to retrieve the same pin arrangement for a component whether it is made with one integration technology or another. This is not the case when the circuits that are integrated into a chip are not limited to a combination of logic circuits but comprise other elements that draw upon specific integration techniques of the technology used, as is the case at present with phase-locked loop oscillators integrated into the periphery of the chips to provide a clock signal. Indeed, it is then difficult to keep the same electrical characteristics for such elements during a change in integration technology. Furthermore, there often rises a problem of the arrangement of the peripheral connection pads as these pads, in most cases, are specialized and have positions that vary with the integration technologies.
Phase-locked loop oscillators however are very useful for the obtaining, with fast technologies, of behavior whose temporal characteristics were obtained hitherto by means of the relative slowness of earlier technologies.
The present situation therefore is one in which the phase-locked loop oscillators form an obstacle to changes in integration technologies which, furthermore, are indispensable to ensuring the permanence of an electronic circuit based on combinations of logic circuits integrated into a chip.
The present invention is aimed at providing a phase-locked loop oscillator whose design facilitates changes in integration technology. To do this, it is proposed to design a phase-locked loop oscillator solely by means of logic circuits so that its integration draws solely upon libraries of logic circuits.